1. Field of the Invention
The present invention relates to a signal transfer device for driving a semiconductor power switch such as an IGBT (Insulated Gate Bipolar Transistor) provided on a high side of a switching power supply, an inverter or any of various drive circuits including a semiconductor switching element.
2. Description of the Background Art
FIG. 11 is a view showing a configuration example of a typical switching power supply constituted by semiconductor power switches one of which includes a signal transfer device transferring a signal through an insulating transformer. For example, an IGBT, an MOSFET (Metal Oxide Field-Effect Transistor) etc. can be used as each of the semiconductor power switches. In a circuit for switching the semiconductor power switches, a high-side semiconductor power switch MH is driven to turn ON/OFF in accordance with an output from the signal transfer device through the insulating transformer.
FIG. 12 is a view showing the configuration of a background-art signal transfer device. In FIG. 12, the background-art signal transfer device is constituted by a transmitting circuit 200, a receiving circuit 300, and an insulating transformer 100 provided between the transmitting circuit 200 and the receiving circuit 300. In addition, a driver 400 is connected to a rear end of the receiving circuit 300. Further, the insulating transformer 100 has a configuration in which two transformer parts, i.e. a transformer part 1 and a transformer part 2, are used so that a signal (set signal) indicating a turn-ON timing of the semiconductor power switch MH can be transmitted to an R1 terminal of the receiving circuit 300 through the transformer part 1 and a signal (reset signal) indicating a turn-OFF timing of the semiconductor power switch MH can be transmitted to an R2 terminal of the receiving circuit 300 through the transformer part 2. As shown in FIG. 12, configuration is made so that output terminals of secondary-side windings of the transformer part 1 and the transformer part 2 can have the same magnetic polarity (voltages in the output terminals can change in the same direction when a magnetic flux changes in one and the same direction). The background-art signal transfer device in FIG. 12 drives the semiconductor power switch MH on the high side of the switching circuit shown in FIG. 11, through the driver 400.
FIG. 13 is a view showing an ideal operation waveform of the background-art signal transfer device shown in FIG. 12. FIG. 14 is an operation waveform view for explaining a problem of the background-art signal transfer device shown in FIG. 12. Since FIG. 14 partially overlaps with FIG. 13, operation of the background-art signal transfer device will be described with reference to FIG. 11 and FIG. 14.
When the high-side power switch MH shown in FIG. 11 is driven, a GND2 potential in FIG. 12 and FIG. 14 fluctuates in accordance with ON/OFF operations of power switches MH and ML. When the power switch MH turns OFF and the power switch ML turns ON due to an output from the signal transfer device, the GND2 potential drops from a high-side power supply voltage to GND1, as shown in FIG. 14. Due to the fluctuation of the GND2, plus common-mode noises (N1+ and N2+) occur in the signal terminals R1 and R2 of the receiving circuit 300 side through parasitic capacitances (not shown).
On the other hand, when the power switch ML turns OFF and the power switch MH turns ON due to an output from the signal transfer device, the GND2 potential rises from the GND1 to the high-side power supply voltage. Due to the fluctuation of the GND2, minus common-mode noises (N1− and N2−) occur in the signal terminals R1 and R2 of the receiving circuit 300 side through the parasitic capacitances (not shown).
In some cases, the high-side power switch MH shown in FIG. 11 may turn ON/OFF by mistake due to any of the aforementioned common-mode noises.
In the background art in order to prevent malfunction from being caused by common-mode noise, a circuit (not shown) for detecting the common-mode noise and suppressing generation of a false pulse is usually mounted inside the receiving circuit 300.
JP-A-2013-51547 discloses a configuration in which a detection circuit for preventing malfunction from being caused by common-mode noise is mounted (see Paragraph [0058], FIG. 5).
JP-A-3-44507 discloses a circuit configuration for preventing malfunction from being caused by common-mode noise (see FIG. 1).
The aforementioned configuration described in JP-A-2013-51547 has a problem that a receiving circuit becomes complicated and a malfunction preventing effect deteriorates as the fluctuation width of GND2 increases.
In addition, the configuration described in JP-A-3-44507 also has a problem that a receiving circuit becomes complicated because a device such as a differential amplifier for canceling common-mode noise by subtraction of a common-mode voltage signal is required on the side of the receiving circuit.
Therefore, an object of the invention is to provide a signal transfer device for transferring a signal through an insulating transformer, in which occurrence of common-mode noise can be suppressed and a countermeasure circuit against the noise can be simplified.